bpo-39943: Add the const qualifier to pointers on non-mutable PyUnicode data. (GH-19345)

diff --git a/Modules/cjkcodecs/multibytecodec.c b/Modules/cjkcodecs/multibytecodec.c
index a09c75d..9f9fbeb 100644
--- a/Modules/cjkcodecs/multibytecodec.c
+++ b/Modules/cjkcodecs/multibytecodec.c
@@ -228,7 +228,7 @@
         Py_ssize_t r;
         Py_ssize_t inpos;
         int kind;
-        void *data;
+        const void *data;
 
         replchar = PyUnicode_FromOrdinal('?');
         if (replchar == NULL)
@@ -457,7 +457,7 @@
     Py_ssize_t finalsize, r = 0;
     Py_ssize_t datalen;
     int kind;
-    void *data;
+    const void *data;
 
     if (PyUnicode_READY(text) < 0)
         return NULL;